To understand complex cognition, neurophysiological studies of large-scale networks of neurons are necessary. While human studies using fMRI have shown evidence for task-related networks across the brain, these studies lack the ability to evaluate mechanisms at the cellular level. Similar brain networks are assumed to support cognition across mammals (Lu et al., 2012), but the most common rodent and primate models present challenges in achieving high-volume multisite single-unit recordings in behaving animals.
The domestic pig (Sus scrofa domesticus) has a large brain and thick skull, allowing for large implants targeting multiple brain regions. It has been shown that domestic pigs can be trained to perform a conditional associative learning paradigm. Behavioral performance of the domestic pig has also been shown to mirror that observed in humans performing the same task.
To fully understand the neural network associated with a behavior, untethered, intracranial electrode probe assemblies are implanted into different regions of a pig brain to obtain large-scale electrophysiological recordings of brain activity during performance of the task. Stereotactic devices are often used to guide and accurately place such intracranial devices. Stereotactic surgical procedures to place these types of devices within a body or tissue can require accuracy to within millimeters or even micrometers. Such accuracy can depend upon the stereotactic device used to guide the intra-body devices to specific points in the tissue, particularly in the brain.
The differences in anatomy, size, orientation of features, and points of attachment necessitate different types of stereotactic devices for different species. Unfortunately, the current methods for placement of intracranial devices in large animals, such as the domestic pig, involve the use of large animal stereotaxic head fixtures, which require tight head restraints that penetrate the skin and contact the skull in multiple locations. This method also requires the head to be leveled and oriented relative to the stereotaxic head fixture.
Once emplaced, electrode assemblies are often permanent or semi-permanent for long term treatment, therapies, or research and often have externally exposed ends protruding outside the skull. These exposed ends need to be supported to ensure that the intra-cranially placed devices remain in position. Those that provide a direct conduit into the brain need to be covered to ensure that undesirable materials are not introduced into the brain. This makes intracranial devices implanted in animals particularly problematic, since normal activities of the animal can cause collisions with the exposed ends and, in some cases, the animal can actively attempt to remove or displace the exposed ends.